Organic electroluminescent (EL) elements include a thin film containing a luminescent organic compound interleaved between a cathode and an anode. Electrons and holes are injected into the thin film where they are recombined to create excitons. Light is emitted by utilizing luminescence (phosphorescence or fluorescence) upon deactivation of excitons. The organic EL elements are characterized by plane light emission at a high luminance of about 100 to 100,000 cd/m.sup.2 with a low voltage of about 10 volts and light emission in a spectrum from blue to red color by a simple choice of the type of fluorescent material.
The organic EL elements, however, are undesirably short in effective life, less durable during storage and less reliable because of the following factors.
(1) Physical changes of organic compounds: Growth of crystal grain domains renders the interface non-uniform, which causes deterioration of electric charge injecting ability, short-circuiting and dielectric breakdown of the element. Particularly when a low molecular weight compound having a molecular weight of less than 500 is used, grains develop and grow, substantially detracting from film quality. Even when the interface with indium tin oxide (ITO) is rough, significant development and growth of grains occur to lower luminous efficiency and allow current leakage, ceasing to emit light. Local dark spots are also formed.
(2) Oxidation and stripping of the cathode: Although metals having a low work function such as Na, Mg, and Al are used as the cathode in order to facilitate electron injection, these metals are reactive with oxygen and moisture in air. As a result, the cathode can be stripped from the organic compound layer, prohibiting electric charge injection. Particularly when a polymeric compound is applied as by spin coating, the residual solvent and decomposed products resulting from film formation promote oxidation reaction of the electrodes which can be stripped to create local dark spots.
(3) Low luminous efficiency and increased heat build-up: Since electric current is conducted across an organic compound, the organic compound is placed under an electric field of high strength and cannot help heating. The heat causes melting, crystallization or decomposition of the organic compound, leading to deterioration or failure of the element.
(4) Photo-chemical and electro-chemical changes of organic compound layers.
With respect to blue light-emitting devices, in particular, there are available few blue light-emitting substances which can provide for reliable stable devices. In general, blue light-emitting substances are highly crystalline. For example, diphenylanthracene has high crystallinity despite a high fluorescent quantum yield. Using this compound as a light emitting material fails to manufacture a reliable device having high luminance and high efficiency. See C. Adachi et al., Applied Phys. Lett., 56, 799 (1990).